1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplex OFDM mobile communication system. In particular, the present invention relates to a method and apparatus in a Node B for assigning sub-carriers to a mobile terminal for data transmission/reception.
2. Description of the Related Art
A signal transmitted on a radio channel arrives at a receiver from different paths because of obstacles between a transmitter and the receiver. The characteristics of the multi-path radio channel are defined by the maximum delay spread and signal transmission period of the channel. If the transmission period is longer than the maximum delay spread, no interference is generated between successive signals and the channel is characterized by frequency non-selective fading in a frequency domain. For high-speed transmission in a wide band, however, the transmission period is shorter than the maximum delay spread, causing interference between successive signals. Thus, a received signal undergoes inter-symbol interference (ISI). In this case, the channel is characterized by frequency selective fading in the frequency domain. A single-carrier transmission scheme adopting coherent modulation requires an equalizer to eliminate the ISI. Also, as the data rate increases, distortion caused by the ISI becomes severe, thereby increasing the complexity of the equalizer. As a solution to the equalizer problem in the single-carrier transmission scheme, Orthogonal Frequency Division Multiplex (OFDM) was proposed.
Typically, OFDM is defined as a two-dimensional access technology comprising Time Division Access (TDA) and Frequency Division Access (FDA). Therefore, each OFDM symbol is transmitted on a predetermined sub-channel composed of distributed sub-carriers.
The orthogonal nature of OFDM allows the spectrums of sub-channels to overlap, having a positive effect on spectral efficiency. Since OFDM modulation/demodulation is implemented by Inverse Fast Fourier Transform (IFFT)/Fast Fourier Transform (FFT), a modulator/demodulator can be efficiently realized digitally. Also, the robustness of OFDM against frequency selective fading and narrow band interference makes OFDM effective for high-speed data transmission standards such as IEEE 802.11a, IEEE 802.16a, and IEEE 802.16b for a large-volume radio communication system.
OFDM is a special case of Multi Carrier Modulation (MCM) in which a serial symbol sequence is converted to parallel symbol sequences and modulated to mutually orthogonal sub-carriers (sub-channels) prior to transmission.
The first MCM systems appeared in the late 1950's for military high frequency radio communication, and OFDM with overlapping orthogonal sub-carriers was initially developed in the 1970's. In view of orthogonal modulation between multiple carriers, OFDM has limitations in actual implementation for systems. In 1971, Weinstein, et. al. proposed an OFDM scheme that applies Discrete Fourier Transform (DFT) to parallel data transmission as an efficient modulation/demodulation process, which was a driving force for the development of OFDM. Also, the introduction of a guard interval and a cyclic prefix as the guard interval further mitigates adverse effects of multi-path propagation and delay spread on systems. This is a reason why OFDM has been widely exploited for digital data communications such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (W-ATM).
Although hardware complexity was an obstacle to widespread use of OFDM, recent advances in digital signal processing technology including FFT and IFFT enable OFDM to be implemented. OFDM, similar to Frequency Division Multiplexing (FDM), provides optimum transmission efficiency in high-speed data transmission because it transmits data on sub-carriers, maintaining orthogonality among them. The optimum transmission efficiency is further attributed to good frequency use efficiency and robustness against multi-path fading in OFDM. Overlapping frequency spectrums lead to efficient frequency use and robustness against frequency selective fading and multi-path fading. OFDM reduces the effects of the ISI by using guard intervals and enables the provisioning of a simple equalizer hardware structure. Furthermore, since OFDM is robust against impulse noise, it is increasingly popular in communication systems.
FIG. 1 is a block diagram of a conventional OFDM mobile communication system. Its structure will be described in detail with reference to FIG. 1.
For the input of bits, a channel encoder 100 outputs code symbols. A serial-to-parallel (S/P) converter 105 converts a serial code symbol sequence received from the channel encoder 100 to parallel symbol sequences. A modulator 110 maps the code symbol to a signal constellation by Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), 16-ary Quadrature Amplitude Modulation (16QAM), or 64-ary Quadrature Amplitude Modulation (64QAM). The number of bits forming a modulation symbol is preset for each of the modulations: a QPSK modulation symbol has 2 bits, a 8PSK modulation symbol has 3 bits, a 16QAM modulation symbol has 4 bits, and a 64QAM modulation symbol has 6 bits. An IFFT 115 inverse-fast-Fourier-transforms modulation symbols received from the modulator 110. A parallel-to-serial (P/S) converter 120 converts parallel symbols received from the IFFT 115 to a serial symbol sequence. The serial symbols are transmitted through a transmit antenna 125.
A receive antenna 130 receives the symbols from the transmit antenna 125. A serial-to-parallel (S/P) converter 135 converts a received serial symbol sequence to parallel symbols. An FFT 140 fast-Fourier-transforms the parallel symbols. A demodulator 145, having the same signal constellation as used in the modulator 110, demodulates the FFT symbols to binary symbols by the signal constellation. The demodulation depends on the modulation. A channel estimator 150 channel-estimates the demodulated binary symbols. The channel estimation estimates situations involved in transmission of data from the transmit antenna, to thereby enable efficient data transmission. A P/S converter 155 converts the channel-estimated binary symbols to a serial symbol sequence. A decoder 160 decodes the serial binary symbols and outputs decoded binary bits.
FIG. 2 illustrates an operation in a Node B for assigning sub-carriers to a User Equipment (UE) in an OFDM mobile communication system. With reference to FIG. 2, sub-carrier assignment to a UE from a Node B will be described below.
Transmission data is modulated in a modulator 200 and transmitted through an antenna 202. As stated, the modulated data is transmitted on a plurality of sub-carriers. The Node B uses all of the sub-carriers or a selected part of the sub-carriers, for transmission of the modulated data.
A feedback information generator 206 estimates the channel condition of data received through a receive antenna 204. The feedback information generator 206 measures the Signal-to-Interference power Ratio (SIR) or Channel-to-Noise Ratio (CNR) of the received signal. That is, the feedback information generator 206 measures the channel condition of an input signal transmitted on a particular channel (or sub-carrier and transmits the measurement to a sub-carrier allocator 208. Table 1 illustrates an example of feedback information that the feedback information generator 206 transmits to the sub-carrier allocator 208.
TABLE 1Sub-carrierFeedback informationSub-carrier #0aSub-carrier #1bSub-carrier #2dSub-carrier #3cSub-carrier #4eSub-carrier #5gSub-carrier #6dSub-carrier #7e. . .. . .Sub-carrier #N − 1f
In the case illustrated in Table 1, data is transmitted on N sub-carriers. Feedback information a to g is an SIR or CNR generated from the feedback information generator 206. The sub-carrier allocator 208 determines a sub-carrier on which data is delivered based on the feedback information. The sub-carrier allocator 208 selects a sub-carrier having the highest SIR or CNR. If two or more sub-carriers are used between the Node B and the UE, as many sub-carriers having the highest SIRs or CNRs as required are selected sequentially. If the SIR or CNR is higher in the order of a>b>c>d>e>f>g, the sub-carrier allocator 208 assigns sub-carriers in the order of sub-carrier #0, sub-carrier #1, sub-carrier #3, sub-carrier #2, and so on. If one sub-carrier is needed, sub-carrier #0 is selected. If two sub-carriers are used, sub-carrier #0 and sub-carrier #1 are assigned. If three sub-carriers are used, sub-carrier #0, sub-carrier #1, and sub-carrier #3 are assigned. If four sub-carriers are used, sub-carrier #0, sub-carrier #1, sub-carrier #3 and sub-carrier #2 are assigned.
The above-described sub-carrier assignment is performed in two stages: one is to arrange feedback information according to channel conditions and the other is to assign as many sub-carriers as needed to a UE based on the feedback information. The feedback information generator measures the channel condition for each sub-carrier and transmits the channel condition measurement to the sub-carrier allocator. However, an existing mobile communication system is limited in the data rate at which uplink data is transmitted. Since the uplink is at a low rate, it is impossible to transmit the measured channel condition information to the Node B on the low-rate uplink. Moreover, when the channel environment varies with time as in a mobile communication system, the sub-carrier assignment must be periodic and that is shorter than a coherence time. However, when the feedback information is delivered on a sub-carrier basis as described before, it takes a long time to transmit the feedback information, which makes it impossible to assign sub-carries to the UE periodically. The transmission of feedback information for each sub-carrier seriously reduces available radio resources. Therefore, techniques to solve these problems are studied.